The present invention relates to a fuel injection system for automotive engines and, more particularly, to a fuel injection system equipped with a thermal type air flow meter.
An electronic fuel control system has been put into practical use in which the rate of fuel supply to an engine is controlled in accordance with a signal representing the intake air flow rate and other signals representing the condition of operation of the engine. This type of electronic fuel control system is taking a growing interest not only from the view point of reduction in fuel consumption but also from the view point of emission control as well.
On the other hand, for a multi-cylinder engine having an injection type fuel supply system, it is considered a better policy from the view point of cost of manufacture to provide only one fuel injector in the gathering portion of the intake manifold or at a point upstream of the throttle valve in the intake system. The known electronic fuel control system usually incorporates a microcomputer which operates to determine the optimum fuel supply rate on the basis of a signal from an air flow meter as well as other signals representing the condition of operation of the engine, e.g., engine speed signal, intake pressure signal, throttle valve opening degree signal, atmospheric pressure signal and so forth. Vane type air flow meters comprising rotatable vanes and air flow meters of Karman's vortex street are now available for use as the air flow meters in the electronic fuel control systems of the kind specified above.
The conventional electronic fuel control system, however, suffers from the following problems arising from the method of metering the intake air flow rate. Namely, since the method conventionally used for metering the air flow rate cannot directly sense the air flow rate in terms of weight, it is not possible to accurately control the air-fuel ratio in response to a change in the atmospheric pressure. In addition, the conventionally used air flow meter has such a large size as to make it difficult to find its mounting space.
It is also to be pointed out that the productivity is inevitably low because the production process has been such that the final confirmation of the performance of the fuel control system as a whole is made after the mounting of the air flow meter and the fuel injection system which are fabricated and administrated separately.
In order to obviate this problems, it has been attempted to use, as proposed in the specification of the U.S. Pat. No. 4,264,961, a hot-wire type sensor having a sensing hot wire disposed in a by-pass air passage formed in a venturi chamber. This attempt, however, encounters the following problem: When the velocity of the intake air flow is increased, the fuel injected by the injector is whirled up around the outlet of the by-pass air passage subjected to the venturi vacuum and is undesirably adhered to the wall of the intake passage to impair the atomization of the fuel. This problem is serious particularly when the engine is operating under a heavy load with a wide throttle opening because, in such a case, the adhesion of the fuel to the intake passage wall is increased due to a heavy whirling of fuel particles caused by the pulsation of the intake vacuum.